Wireless network infrastructure equipment is increasingly being used to allow computing devices to communicate over a wireless medium to a wired network such as the Internet. In a wireless data network, a plurality of local computing devices, such as PCs, are supported via wireless subscriber access units. Each subscriber access unit (SAU) provides a wireless radio link to a base station processor. The base station processor (BSP) is also connected to an Internet gateway that provides a connection to a wired network.
Wired networks typically employ congestion control techniques to detect the speed with which messages are propagated across the network to a recipient. These techniques reduce congestion through avoiding overburdening a recipient with messages by reducing the rate at which messages are sent, and consequentially reducing throughput. However, normal operation of the wireless network exhibits different latencies than those exhibited during normal operation of the wired network. Accordingly, such techniques can interpret the queuing of messages at the base station processor as congestion, and accordingly, reduce throughput. In general, the protocols employed in the wired network do not lend themselves well to efficient communication over wireless connections.
In a TCP/IP network, for example, congestion control techniques such as slow start and congestion avoidance are employed. In accordance with the slow start technique, as defined in Internet RFC 2581, an acknowledgment message (ACK) is expected as a response to every second packet (message) sent. A sliding window protocol is employed to regulate the number of unacknowledged messages which can be outstanding at any time. This sliding window, which is indicative of the number of unacknowledged messages permitted at any time, is initially set at a low number, typically two messages. The number of messages permitted in the window is gradually increased as the ACKs are received in a timely manner. If, however, an ACK is not received after a timeout threshold, or if duplicate ACKs are received, the window may be reset to the initial value (typically one) and must again be permitted to gradually increase as described above.
The queuing of messages at the base station processor, however, is not necessarily indicative of congestion in the wireless network. Rather, the queuing is indicative of the propagation delay or the assignment delay of wireless resources. This propagation delay is interpreted, however, as congestion by the wired line protocols such as TCP/IP when the ACK is not received within the timeout expected by the wired network. Accordingly, the wireless connection tends to be throttled back to a sliding window of two by slow start more frequently, thereby reducing throughput.
Another TCP/IP congestion control parameter employed by congestion avoidance and slow start is an advertised window. The advertised window is contained in the ACK and informs the sender how many more messages the receiver can accept. This prevents a sender from overburdening a receiver with more packets than it can buffer, therefore avoiding sending packets which are likely to be dropped or result in timeout. While technically separate, the slow start and congestion avoidance mechanisms are typically implemented in a complementary manner. The sliding window and slow start are congestion control imposed by the sender, while the advertised window is congestion control imposed by the receiver.
A further aspect of the advertised window mechanism is a persist mode. When a receiver can accept no more data, it sends a message having an advertised window of zero. This transmission has the additional effect of preventing the sender from sending any more data until the receiver sends another ACK message with a nonzero advertised window. Persist mode, therefore, allows the receiver to “pause” the sender until more messages can be processed at the receiver side. However, an ACK indicating an advertised window of zero does not trigger slow start and reset the sliding window.
It would be beneficial, therefore, to provide a system and method for determining when a connection including a wireless link is about to experience a timeout, and sending a suppression message to trigger a persist mode to avoid slow start from resetting the window size, while maintaining the same TCP/IP end-to-end connection between the sender and receiver to avoid tearing down and buffering messages to accommodate the wireless latency.